1. Field of the Invention
The present invention relates to a gas sensor for measuring a content of nitrogen oxide and oxygen, contained in a combustion exhaust gas.
2. Description of the Background Art
As a gas sensor for measuring a content of nitrogen oxide and oxygen, contained in a combustion exhaust gas, a sensor is known, which has a structure constituted of a first chamber for pumping oxygen and a second chamber relating to measurement of nitrogen oxide, and which measures an oxygen concentration in the first chamber, and simultaneously reduces the oxygen concentration in a measurement gas up to about 10 ppm, decomposes by reduction the nitrogen oxide in the second chamber, detects the generated oxygen by pumping in the similar manner, and measures a concentration of a nitrogen oxide gas, as described in, for example, JP-A-10-90220, Japanese Patent 2,885,336, and Kogyo Kanetsu (Industrial Heating), vol. 41, No. 6, pp. 31-36. Those sensors are produced by bonding thin plates of yttrium oxide-added zirconium oxide (hereinafter referred to as “YSZ” for brevity) sintered body.
Further, a sensor is known, wherein a substrate comprises a YSZ film formed on a trisilicon tetranitride (Si3N4) (hereinafter referred to as “silicon nitride” for brevity) porous sintered body comprising needle crystals, as described in, for example, JP-A-2000-62077.
The sensor having such a structure achieves to shorten a temperature rising time due to lowering in measurement temperature and improvement in thermal shock resistance.
However, the sensors described in JP-A-10-90220, Japanese Patent 2,885,336, and Kogyo Kanetsu (Industrial Heating), vol. 41, No. 6, pp. 31-36 each use a thick plate material. As a result, heating at 700° C. or higher is required, and it takes 15 minutes or longer for temperature rising, and further, it takes 30 minutes or longer until a stable output is obtained. In addition, the structure of bonded sintered bodies generally has the problem that breakage may occur at bonded portions by repeating temperature rising and cooling.
The above-described conventional sensors each are constituted of two parts, a part for exhausting oxygen gas and a part for measuring NOx gas. Therefore, where exhaustion of oxygen gas in the pre-stage is not sufficient, or there is scattering in exhaustion, the residual oxygen leads to an error in measurement of NOx at the NOx measurement part, and the problem occurs in measurement accuracy of ppm order.
The sensor described in JP-A-2000-62077 involves gas diffusion from a thickness direction. Thus, function as a gas diffusion layer is not sufficient, and there is the problem that accuracy of quantification deteriorates.